Among 551 consecutive recipients of allogeneic bone marrow transplants, 451 survived more than 3 months and were evaluated for chronic graft-versus-host disease (GVHD). Most of the donors were HLA-identical siblings or parents (n = 334). Patients with HLA-mismatched donors (n = 30) and matched unrelated donors (MUD) (n = 87) were also included in the study. In the analysis of all patients, the 5-year cumulative incidence of chronic GVHD was 45%. We analysed 34 risk factors. High recipient age was the single most important risk factor (P < 0.001). Other significant risk factors in multivariate analysis were: acute GVHD grades I-IV (P < 0.001), immune female donor to male recipient (P = 0.006) and chronic myelogenous leukaemia (CML), compared with all other diagnoses (P = 0.014). The cumulative 5-year incidence of chronic GVHD, with no significant risk factors present, was 9%, 29% with one risk factor, 53% with two, 68% with three and 75% with all four risk factors present. In patients with HLA-identical sibling donors and GVHD prophylaxis consisting of a combination of methotrexate (MTX) and cyclosporin A (CsA) (n = 208), increasing recipient age (P < 0.001) and CML (P = 0.007), were found to be significant risk factors for chronic GVHD. Finally, a multivariate analysis in recipients of bone marrow from unrelated donors (n = 89) showed recipient age alone (P = 0.006) to be significantly associated with chronic GVHD.